1. Field of the Invention
The present invention relates to a servo control system for an optical disk device.
The present invention further relates to a focus offset correction circuit of an optical pickup device.
2. Description of the Related Art
In general, with respect to an optical disk device, it is important to converge the optical scanning beam to irradiate an optical disk surface in a state that the spot of the beam is focussed on a track of the disk. For this purpose, the focussed point of the beam is always controlled by a focussing/tracking servo control system.
An example of such a servo control system is disclosed in the papers of Optical Memory Symposium, Sept. 21, 1988, page 77 to 78, titled "Digitization of Servo System for Optical Disk". This is aiming to realize a digitized intelligent servo system which constitutes a control system of the optical disk device.
With regard to one of the servo systems, for example, a focussing servo system, the construction and function of the system are as follows.
First, the reflection rays reflected from the optical disk are received by a photoelectric detector and converted to electric signals. The detector is divided to at least two detection portions. The outputs from the two detection portions are passed through a low pass filter, respectively, and input to an adder which adds the two output signals to form an added up signal. Also, the two output signals from the two detection portions are input to a subtracter after passing through the respective low pass filters. The subtracter subtracts one of the signals from the other to form a difference signal corresponding to the difference between the two signals. The difference signal is used as a focus error signal which corresponds to the amount of dislocation of the object to be controlled from a desired set point. In this case, in order to maintain the gain of the error signal constant irrespective of the change or fluctuation of the optical amount of the reflection rays from the optical disk, the difference signal is converted to a digital signal by an A/D converter using the added up signal from the adder as the reference input signal. Thereby, the difference signal is always normalized by the added up signal corresponding to the optical amount of the reflection rays, which makes it possible to perform an AGC (Automatic Gain Control) function.
The error signal which was digitized by the A/D converter is introduced to a logic circuit which is controlled by a CPU. After that, the error signal is again converted to an analogue signal by a D/A converter. The analogue signal output from the D/A converter is input to a focus servo control circuit which comprises a phase correction circuit and an objective lens driving circuit, so that a servo control loop is formed.
In the above mentioned structure, it is necessary to correct the offset of the error signal which offset means that the desired set point is shifted and dislocated from a reference point of the servo system caused by dimensional errors of the optical system or change of the ambient temperature. For this purpose, a corrective analogue signal derived from another D/A converter which is controlled by the CPU is added to the error signal by another adder so as to cancel the offset of the error signal.
It is to be noted that the low-pass filters function to minimize the noise generated and transmitted backward from the A/D converter at the time of sampling the signals.
Also, the logic circuit functions to generate a timing signal for stopping the servo operation and an error discrimination signal.
The above mentioned circuit structure has to be prepared for every control system. For example, a focus control system comprises one special control circuit having the above mentioned structure for its own. Therefore, with regard to the focus/tracking control system, it is necessary to prepare another similar circuit for the tracking control system in addition to that for the focus control system, which makes the control system large and complicated as a whole.
Besides, the above mentioned circuit structure has a problem as follows.
In general, an A/D converter quantizes the potential difference between a positive reference input signal (RT) and a negative reference input signal (RB) to 2.sup.n amount units wherein "n" designates the number of bits so as to digitize the input voltage of the corresponding signal.
On the other hand, the servo error signal has a voltage of predetermined value (for example, zero) when the signal is coincident with the servo set point (desired point), that is when the beam spot is in focus on the desired track, for example. Also, when the signal value becomes discordant from the set point, the voltage of the signal becomes positive or negative according to the plus side or minus side of the signal with respect to the set point.
Therefore, when the error signal is to be converted to a digital signal by an A/D converter, the set point of voltage is arranged as a center point of the positive reference voltage (RT) and the negative reference voltage (RB), that is, a point of (RT+RB)/2 so as to obtain a sufficient dynamic range. In this case, in order to perform a reliable AGC operation, when the added up signal is input as the references (RT) and (RB), it is necessary to prevent the set point (RT+RB)/2 from being shifted and dislocated from the central point of the voltage along with the change of the references (RT) and (RB). Therefore, it is necessary to symmetrically change the references (RT) and (RB) with respect to each other relative to the central set point (RT+RB)/2. In other words, when a positive signal is input to the reference (RT), a negative signal of the same value has to be input to the reference (RB). For this purpose, an inversion amplifier of gain 1 is used to generate the signal having an inverted polarization. In this case, if the inversion amplifier has an offset or a gain error, the references (RT) and (RB) do not change symmetrically, which results in that the central set point of voltage for servo control of the error signal changes. It is possible to avoid such a problem by properly adjusting and correcting the offset and the gain of the amplifier. However, it is troublesome and costly to adjust and correct the amplifier.
In an optical pickup device, a laser beam which is converged by an objective lens is irradiated to an optical disk (or photomagnetic disk). In this case, it is necessary to adjust the position of the objective lens to focus the beam spot on the disk surface. Such an adjustment of the position of the objective lens in the focussing direction is carried out by a focus servo control circuit in response to the focus error signal.
Most desirably, when the focus error signal is zero, the beam is focussed on the disk surface so that the beam spot diameter is minimized thereon and that the amplitude of the track error signal (or information signal) which is used for tracking servo control is maximized.
However, in actual cases, the maximum point of the track error signal does not necessarily coincide with the zero point of the focus error signal due to the constructional errors of the optical system for detecting the focus error signal and the temperature change around the system. Therefore, the offset of the focus error signal has to be corrected so that the maximum point of the track error signal coincides with the zero point of the focus error signal.
The correction of the offset is carried out in such a way that an offset voltage is applied to the focus error signal by an offset voltage applying means so that the focus error signal becomes zero at the point where the amplitude of the track error signal becomes maximum.
In the above mentioned offset correcting operation, the offset voltage to be applied to the focus error signal corresponding to the focus set point (zero cross point) is determined on the basis of the slope angle or inclination (gain) of the focus error signal at the zero point thereof under the assumption that the focus error signal is linear.
For example, under the assumption that the desired set point for the focal position of the track error signal corresponding to the maximum point of the track error signal waveform is positioned at a point dislocated from the zero point by a certain distance, the offset amount at the set point is determined from the assumptive linear line passing the zero point.
However, actually, the focus error signal is not necessarily linear. Therefore, if the offset amount based on the assumptive linear line is applied to the focus servo circuit, the focal position is dislocated from the set point. The amount of this dislocation increases according as the set point for the focal position is separated remote from the zero point. If the dislocation amount exceeds the allowable range for the focus error signal, the focus servo system becomes inoperative.
As a result, the offset voltage is limited within a small range so as not to make the servo system inoperative, which results in that the offset correction circuit can not cover the entire range of the set point of the focus offset.
Also, if a large amount of the offset voltage is applied to the focus servo system, since the focus error signal is not linear, that is the slope angle (gain) of the focus error signal is actually smaller than that of the assumptive linear line at the zero cross point, reliability of the servo control is impaired.